ath9k: remove ath9k_mod_tsf64_tu

Remove ath9k_mod_tsf64_tu() function while we could
use div_u64_rem() function.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>